A dumper is intended to run over site or mine paths or roads that are somewhat twisty and partially covered with stones of various sizes. In use, the tires of a dumper are subject on the one hand to the mechanical loadings of running and, on the other hand, to the mechanical loadings generated by the stones spread over the road. Amongst the mechanical loadings of running the transverse loadings resulting from a somewhat twisty journey, are particularly significant to such a tire and require the tire to be able to generate sufficient transverse thrust. Furthermore, the mechanical loadings generated by the stones create impacts on the crown of the tire which are liable to damage the tire: the crown of the tire has therefore to be capable of absorbing these impacts.
A tire comprises a crown intended to come into contact with the ground via a tread. This crown is connected by two side walls to two beads which are intended to provide the mechanical connection between the tire and the rim on which it is mounted.
Because a tire has a geometry exhibiting symmetry of revolution about an axis of rotation, the geometry of the tire is generally described in a meridian plane containing the axis of rotation of the tire. For a given meridian plane, the radial, axial and circumferential directions respectively denote the directions perpendicular to the axis of rotation of the tire, parallel to the axis of rotation of the tire and perpendicular to the meridian plane.
In what follows, the expressions “radially on the inside of or, as the case may be, radially on the outside of” mean “closer to or, as the case may be, further away from, the axis of rotation of the tire”. “Axially on the inside of or, as the case may be, axially on the outside of” mean “closer to or, as the case may be, further away from, the equatorial plane of the tire”, the equatorial plane of the tire being the plane passing through the middle of the tread of the tire and perpendicular to the axis of rotation of the tire.
A radial tire comprises a reinforcement consisting of a crown reinforcement radially on the inside of the tread, and of a carcass reinforcement radially on the inside of the crown reinforcement.
The carcass reinforcement of a radial tire for a heavy vehicle of the civil engineering type usually comprises at least one carcass layer made up of generally metal reinforcers coated in an elastomeric coating material or coating compound. In the field of tires, an elastomeric material obtained by blending components of the material is usually referred to as a compound. The carcass layer comprises a main part, connecting the two beads of the tire to one another and wound, within each bead, from the inside towards the outside of the tire, around a generally metal circumferential reinforcing element referred to as a bead wire to form a turnup. The metal reinforcers of a carcass layer are substantially parallel to one another and make with the circumferential direction an angle of between 85° and 95°.
The crown reinforcement of a radial tire for a heavy vehicle of the civil engineering type comprises a superposition of crown layers arranged circumferentially and radially on the outside of the carcass reinforcement. Each crown layer is made up of generally metal reinforcers which are parallel to one another and coated in an elastomer coating material or coating compound.
Among the crown layers, a distinction is usually made between the protective layers, which make up the protective reinforcement and are radially furthest towards the outside, and the working layers, which make up the working reinforcement and lie radially between the protective reinforcement and the carcass reinforcement.
The protective reinforcement, made up of at least one protective layer, essentially protects the working layers from mechanical or physico-chemical attack likely to spread through the tread radially towards the inside of the tire.
In the case of a tire for a dumper, the protective reinforcement often comprises two radially superposed and adjacent protective layers. Each protective layer comprises elastic metal reinforcers which are parallel to one another and form with the circumferential direction an angle generally of between 15° and 35° and preferably of between 20° and 30°. The metal reinforcers of the protective layers are usually crossed from one protective layer to the next.
The working reinforcement, made up of at least two working layers, has the function of hooping the tire and of providing the tire with stiffness and road holding. It absorbs both the mechanical loadings of inflation, which are generated by the pressure to which the tire is inflated and transmitted via the carcass reinforcement, and the mechanical loadings of running, which are generated by the running of the tire on the ground and transmitted by the tread. It needs also to withstand oxidation and impact and puncturing, by virtue of its own intrinsic design and that of the protective reinforcement.
In the case of a tire for a dumper, the working reinforcement usually comprises two radially superposed and adjacent working layers. Each working layer comprises inelastic metal reinforcers which are parallel to one another and form with the circumferential direction an angle generally of between 15° and 45°. The metal reinforcers of the working layers are usually crossed from one working layer to the next.
Furthermore, in the case of a tire for a dumper comprising a working reinforcement with two working layers, the angle referred to as the equilibrium angle of the working reinforcement, which is defined such that the square of the tangent of the equilibrium angle is equal to the product of the tangents of the respective angles of the reinforcers of each working layer, is at most equal to 45°. In other words, the equilibrium angle is the angle formed by the metal reinforcers of each of the two working layers of a working reinforcement mechanically equivalent to the reference working reinforcement. This equilibrium angle criterion embodies the fact that the axial or transverse stiffness of the working reinforcement, namely the axial force that has to be applied to the working reinforcement in order to obtain an axial movement of 1 mm, is somewhat high.
As far as characterizing metal reinforcers is concerned, a metal reinforcer is mechanically characterized by a curve representing the tensile force (in N) applied to the metal reinforcer, as a function of the relative elongation (in %) of the metal reinforcer, referred to as a force-elongation curve. Mechanical characteristics in tension such as the structural elongation As (in %), the total elongation at break At (in %), the force at break Fm (maximum load in N) and the breaking strength Rm (in MPa), are deduced from this force-elongation curve, these characteristics being measured in accordance with the 1984 Standard ISO 6892.
The total elongation at break At of the metal reinforcer is, by definition, the sum of the structural, elastic and plastic elongations thereof (At=As+Ae+Ap). The structural elongation As results from the relative positioning of the metal threads that make up the metal reinforcer under a low tensile force. The elastic elongation Ae is the result of the very elasticity of the metal of the metal threads of which the metal reinforcer is made up, considered individually (Hooke's law). The plastic elongation Ap results from the plasticity (irreversible deformation beyond the elastic limit) of the metal of these metal threads considered individually. These various elongations and the respective significance thereof, which are well known to those skilled in the art, are described for example in documents U.S. Pat. No. 5,843,583, WO2005/014925 and WO2007/090603.
An extension modulus (in GPa) is also defined at every point of the force/elongation curve and represents the gradient of the straight line tangential to the force-elongation curve at this point. In particular, the tensile elastic modulus or Young's modulus is the name given to the tensile modulus of the elastic linear part of the force-elongation curve.
Among the metal reinforcers a distinction is usually made between elastic metal reinforcers such as those used in the protective layers and inelastic metal reinforcers such as those used in the working layers.
An elastic metal reinforcer is characterized by a structural elongation As at least equal to 1% and a total elongation at break At at least equal to 4%. Furthermore, an elastic metal reinforcer has a tensile elastic modulus at most equal to 150 GPa and usually of between 40 GPa and 150 GPa.
An inelastic metal reinforcer is characterized by a relative elongation, under a tensile force equal to 10% of the breaking force Fm, at most equal to 0.2%. Moreover, an inelastic metal reinforcer has a tensile elastic modulus usually of between 150 GPa and 200 GPa.
Under the effect of the mechanical loadings generated by the stones present on the ground or impacts, the abovementioned crown reinforcement comprising a protective reinforcement comprising at least two protective layers and a working reinforcement comprising at least two working layers may experience a partial or even total breakage. In practice, the crown layers each yield in turn, from the radially outermost crown layer to the radially innermost crown layer.
In order to characterize the breaking strength of a tire crown reinforcement subjected to impacts, the person skilled in the art knows to carry out tests that involve running a tire, inflated to a recommended pressure and subject to a recommended load over a cylindrical indenter or obstacle of a diameter of between 1 inch, namely 25.4 mm, and 2.2 inches, namely 55.9 mm, according to the size of the tire, and of a set height. The breaking strength is characterized by the critical height of the indenting tool, namely the maximum height of the indenting tool that causes complete breakage of the crown reinforcement, namely that causes all the crown layers to break.